The National Heart, Lung and Blood Institute states that valves located between the atria and ventricles of the heart are responsible for stopping blood backflow. These valves are known as the tricuspid and mitral valves.
The institute further explains that the tricuspid valve is located on the right side of the heart, while the mitral valve is located on the left side. When the ventricle contracts, these valves close to prevent blood from leaking out to the atrium. When the atria contract, the valves open to allow blood to flow into the ventricle. When the valves close, they make a "lub" sound, for which a doctor listens during a medical exam.